Top grade golf balls sold in the United States generally comprise a central core with one or more cover layers formed thereover. A golf ball cover is particularly influential in effecting the compression (feel) and durability (i.e., impact resistance, etc.) of the resulting ball. Various cover compositions have been developed in order to optimize desired properties of the resulting golf balls.
FIGS. 1 and 2 illustrate a conventional golf ball 10 having a single cover layer 14 molded about a golf ball core 16. FIG. 2 illustrates (in an exaggerated view) stress lines 12 extending partially, or entirely across the thickness of the cover layer 14. Stress lines 12 typically result in a crack or fracture across the thickness of the golf ball cover. FIGS. 1 and 2 illustrate one problem that may occur when a very thick, single layer cover is formed about a golf ball core.
Although not wishing to be bound to any particular theory, it is believed that stress lines in a golf ball cover, such as stress lines 12 in cover 14, result from repeated strikes with a golf club, particularly drivers, and temperature effects. Stress lines often serve as initiation sites for crack or fracture propagation in a golf ball cover material. Such cracks or fractures, and their related stress lines, are undesirable in golf ball covers. Moreover, it is particularly undesirable for such stress lines and the resulting cracks or fractures to extend across the entire thickness of a golf ball cover since such damage significantly impairs golf ball performance. And, such cracks or fractures greatly reduce the durability of a golf ball cover.
When a multi-layer cover is employed, each cover layer traditionally has had a significantly different Shore D hardness than an adjacent cover layer in order to impart to the golf ball a particular desired combination of spin and distance characteristics. Although the use of a multi-layer cover configuration reduces the tendency of stress lines, and thus cracks and fractures, propagating across the entire thickness of the cover, such multi-layer arrangement of cover materials, each having its own particular set of properties and characteristics, has associated design and manufacturing problems.
For instance, in order to produce a multiple cover layer golf ball that exhibits a desired set of performance characteristics, it is necessary to design and anticipate an overall performance profile for the set of cover layers. This involves analyzing each of the individual cover layers and any and all effects between the individual cover layers. Even if such daunting design analysis is performed, the increased number of variables may lead to unanticipated difficulties in manufacturing or with the final product golf ball.
In addition, although, once again, not wishing to be bound to any particular theory, it is believed that although a multiple cover layer configuration may reduce the tendency for cracks or fractures that extend through the entire thickness of the cover, such configuration may lead to an increase in the number or frequency of fractures, particularly in applications in which the various cover layers constituting the multi-layer cover each have different physical properties such as hardness and flexural characteristics.
Accordingly, there is a need for an improved golf ball which is less susceptible to cracking or fracturing across the thickness of the cover than currently known single cover layer golf balls. And, there is a need for an improved multiple cover layer golf ball that is simpler to design and manufacture, and which is less susceptible to cracking or fracturing of one or more of the individual cover layers that constitute the multiple cover layer of the ball.